Does the fan and the exterior vent grille really need to be above the level of the bathrooms or can they be lower? I have a bath in the basement and one on the main level. I can't go with the fan above the first level for various reasons and it's hard to do it on the first level as well. Would it be a good idea for that inline fan and the exterior grill to be at the level of the basement (between the floors essentially)? This means that the upper bathroom vent pipe will have to go down to that level. Is it a bad idea?

2 Answers
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Hot air rises so the fan would be more efficient if it were in the ceiling or high up on the wall, but even if it's lower down the wall it should be able to remove most of the moist air satisfactorily.

The whole idea is that air is forced out by the fans. The level of the exit does not need to be above the fan unit. As far as connecting two fans to one exterior vent opening, it will work fine if you install a one way baffle or back flow preventing spring loaded flapper. Some fan units come with this one way baffle factory installed to prevent outdoor air from entering. Fan forced air will take the path of least resistance. You certainly don't want air from one bathroom being pumped into the other bathroom, thus back flow prevention is a must.

Yes, there will be backflow prevention. Fantech offers these: bathroomfanexperts.com/… My main concern going down (about 8 feet) with the pipe rather than horizontal or up. Looks like that's not going to be a huge problem.
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Peter QMay 8 '11 at 12:35

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just be sure the fan is powerful enough for the length of exhaust pipe. Direction won't make that much difference. Remember, the back flow preventer needs to be at each fan location, not the external vent cover as pictured in your link, even though one at the exit location is good in addition to the ones at the individual fans.
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shirlock homesMay 8 '11 at 12:52